Having determined the structure of crystals of deoxy sickle cell hemoglobin at a resolution of 3A it is proposed to refine the model so that it calculated diffraction pattern best agrees with the experimentally observed one. At the conclusion of the refinement the intermolecular contacts will be examined and catalogued. Having obtained another new crystal form of deoxy Hb S, it is proposed to determine its structure by the same molecular replacement methods already employed originally.